1. Field of the Invention
This invention relates to a gallium nitride semiconductor light-emitting device for emitting blue light and short-wavelength-region light and a method of manufacturing the device.
2. Description of the Related Art
A gallium nitride semiconductor (Al.sub.x Ga.sub.1-x N:0.ltoreq..times..ltoreq.1, hereinafter called "GaN") is currently regarded as a remarkable material for blue-light-emitting diodes and short-wavelength-region light-emitting devices.
Since a low-resistance p-type crystal cannot be obtained from GaN, an emitting diode using GaN has a so-called MIS-type structure consisting of a metal electrode (M layer), a semiinsulating GaN layer (I layer) and an N-type GaN layer (S layer). For developing such a light-emitting diode, it is essential to establish a technology of forming an electrode to be connected to an N-type GaN layer. Thus in a light-emitting device using a semiconductor of other III, V group compound such as Al.sub.x Ga.sub.1-x As, the lower-layer electrode is formed on a conductive substrate; to the contrary, in the case of GaN, since sapphire of the substrate is insulating, it is impossible to form the electrode, which is to be connected to the N-type GaN layer, on this insulating substrate.
Since GaN is chemically very stable, it is impossible to partially remove the I-type GaN layer to expose the N-type GaN layer by chemically etching with chemicals and is also impossible to form an electrode on the exposed surface. Further when the electrode portion is provided on the side surface of the N-type GaN layer and wired with metal wires, the thickness of the N-type GaN layer can have only several .mu.m to several tens .mu.m so that the electrode would be poor in reliability and mass-productivity. To this end, following methods have heretofore been reported as technology of forming electrodes on the N-type GaN layer.
The first method is a method of forming a low-resistance region on a part of the GaN layer by treating the substrate with a kind of process. Specifically, as shown in FIGS. 5A through 5C of the accompanying drawings, when epitaxial growth of GaN is made after a bumpy region 12 having such as shallow grooves or scratches is formed on a part of a sapphire substrate 10 by a scriber or a dicer (FIG. 5A), an N-type low-resistance region 18 rather than an I-type layer will be formed (FIG. 5B) even if a large amount of zinc is doped to the GaN layer grown on the bumpy region 12. When an N-side electrode 20 is formed on the low-resistance region 18, a contact with an N-type GaN layer 14 can be made via an N-type low-resistance region 18 (FIG. 5C). Further an I-side electrode 22 is formed on an I-type GaN layer 16 isolatedly from the N-side electrode 20.
Also when epitaxial growth of GaN is made after dielectric films of SiO.sub.2, Al.sub.2 o.sub.3, Si.sub.3 N.sub.4, etc. are heaped on a part of the sapphire substrate 10, likewise the case of forming of the bumpy region 12, an N-type low-resistance GaN layer will be formed on these films so that a contact with the N-type GaN layer can be made by forming an N-side electrode on the N-type low-resistance GaN layer.
The second method is a method of removing an I-type GaN layer such as by dry etching. Specifically, as shown in FIGS. 6A through 6C, an opening 28 is formed after an SiO.sub.2 film 24 is heaped on the I-type GaN layer 16 (FIG. 6A), then the I-type GaN layer 16 directly under the opening 28 is removed by dry etching using a gas such as of CCl.sub.4 or CCl.sub.2 F.sub.2 to expose the N-type GaN layer 14 to thereby form a contact hole 26 (FIG. 6B) so that a contact with the N-type GaN layer 14 can be made if an N-side electrode 20 is formed at the contact hole 26.
Likewise, an opening is formed after an SiO.sub.2 film is heaped on the I-type GaN Layer, and a heating process is performed in a mixed gas atmosphere containing hydrogen chloride and argon in a hydrogen atmosphere at a mixture ratio of 3:1 so that the I-type GaN layer of the exposed surface is discomposed and removed to expose the N-type GaN layer, where an N-side electrode can be formed.
In another method, the I-type GaN layer is mechanically removed by scribing using a diamond needle, and then an N-side electrode is formed there.
However, in the first method, since the N-type low-resistance region 18 is formed by utilizing that GaN grown at the bumpy region 12 on the surface of the substrate 10 will be a polycrystal, it is difficult to control the carrier concentration accurately with good reproducibility. Further since the low-resistance region 18 is formed so as to extend from the surface of the I-type GaN layer 16 to the sapphire substrate 10, it is impossible to set the depth to a desired value. This method is a method that was invented in the manufacture of a GaN crystal by Ga--HC1--NH.sub.3 hydride vapor phase epitaxy (HVPE) previously in use, and cannot be simply used in the production of a crystal by metal organic vapor phase epitaxy (MOVPE) now in use. Particularly by MOVPE process, it is impossible to grow a polycrystalline GaN layer uniformly on a dielectric film such as of SiO.sub.2 or Al.sub.2 O.sub.3 so that a low-resistance region cannot be formed.
The second method requires a multi-step complex process for forming an SiO.sub.2 layer as a mask and its pattern, and dry etching or heating, and for forming an electrode. In the method in which an I-type GaN layer is scribed, the GaN layer would be encountered with a stress to cause a crack, which would be a cause for yield reduction, such as deterioration of electrical characteristics, in the actual device manufacturing.